The urgency to replace petroleum-based plastics and related products with ‘green’ materials has risen exponentially in the past couple of decades. This is mainly due to concerns about fast depletion of petroleum resources, disposal of plastics at the end of their life and ever-increasing landfill costs. Large industries such as aerospace, automotive and construction are the primary contributors to these environmental issues due to production of petroleum-based polymers/plastics and composites for use in structural and high-temperature applications. Using plant-based green and fully sustainable materials to replace conventional plastics would create a significant positive impact on the environment. In the present study, a rosin-based green epoxy (RGE) resin was developed using rosinic acid derived from rosin or colophony, a raw material found in conifers such as pine trees. The RGE resin was used to prepare unidirectional continuous flax fiber reinforced composites. The RGE resin partly containing 0% to 20% DER 732 plasticizer (PL) was crosslinked using tetraethylenepentamine (TEPA) to obtain a rigid thermoset epoxy resin with improved toughness. Flax fibers were modified using a combination of chemical, mechanical and thermal treatments to improve their Young’s modulus and tensile strength from 52 GPa and 955 MPa to 84 GPa and 1261 MPa, respectively. Hand lay-up technique was adopted to fabricate the composites. The flax/RGE-10PL composites with 10 wt% DER 732 plasticizer (w.r.t. RGE resin) and 35 wt% modified flax fibers showed Young’s modulus and tensile strength of 7 GPa and 166 MPa, respectively. The flexural modulus and flexural strength of these composites were 16.3 GPa and 137 MPa, respectively. The RGE resin with 10 wt% DER 732 also displayed a high Tg of 161ºC. With excellent mechanical properties and high Tg, flax/RGE-10PL composites can be considered as potential candidates to replace currently used petroleum-based composites used in secondary structural and moderately high-temperature (80ºC to 150ºC) applications.